Process of purifying phthalic anhydride



d treatment of Patented Feb. 14, 1933 UNITED STATES! PATENT orr cs PAUL C. BOWEBS, 0F PENNS GROVE, NEW JERSEY, ASSIGNOB TO E. I. DU POINT DE NEKOURS & COMPANY, OF WILMINGTON, DELAWARE, A OOBPOBATIOIT OF DELA- WARE PROCESS OF PURIFYING PHTHALIC ANHYDBIDE Io Drawing.

This invention relates to the purification of phthalic anhydride which has been preared by the catalytic oxidation of naphthaene, and it relates more particularly to the hthalic anhydride by heating with a con ensing agent to convert volatile impurities into non-volatile form, so that the phthalic anhydride may be recovered by suitable vaporization.

This application is a continuation in part; the parent application being copending application Serial No. 597,417 filed by applicant on October 27,1922, which has matured into Number 1,728,225 granted September 17 1929.

l have discovered that the volatile im-' purities which are ordinarily resent in crude phthalic anhydride prepare by the vapor phase oxidation of naphthalene in the presence of a catalyst, are readily converted by certain condensing agents into relatively non-volatile substances. Among the impurities in this type of process may be mentioned quinones or quinonic bodies, and the condensing agents suitable for the purification comprise certain acid bodies containing the S0. or HSO, roup. cable to the urification of phthalic an ydride either y distillation under reduced pressure or by sublimation.

If the crude phthalic anhydride is subjected to distillation, either at atmospheric pressure or under vacuum without the pres ence of one of the condensing agents mentioned, the product, although improved by distillation, shows some color, either pink, yellow, or gray, in the molten condition, or when allowed to solidify in bulk, even though it may appear perfectly white when ound. If, however, the crude phthalic anhy ride is distilled in the presence of a small amountof one of the above-mentioned condensing agents, the distilled product is almost or entirely free from color either when molten or whenfallowed to solidify in compact form. It would be diflicult to determine quantitatively the difference between the two distilled products, but the color which shows in the product distilled without a condensing agent also causes difiiculty in certain operations to 1.0% of the weig t of the crude.

The new process is ap li- .above specific examples are considered pref- Appllcation fled August 24, 1929. Serial No. 388,287.

which'involve a reaction with phthalic anhydride.

The process should preferably be carried out about as follows: The crude phthalic anhydride is melted and agitated. The condensin to hol agent is added in amount sufficient back the im urities, generally 0.25

treated crude material is (I) either distilled (or sublimed) directly; or is (2) first heated, with agitation, preferably to about its boiling point at atmospheric pressure, the crude phthalic anhydride transferred into the still or sublimer, and distillation of sublimation performed at either atmospheric .or reduced pressure.

.Bodies containing the SO, radical or the HSO, group comprise acid sodium sul hate acid otassium sulphate sulphuric aci and the like, which have a pronounced acidic character. These are used in a manner similar to the process above described. Various sulphonic acids, and the like, which have a pronounced acidic character, may also be used in a manner similar to the process above described. a

Although the conditions set forth in the erable, it will be understood that various changes may be made in these conditions of The a operation; thus, after the condensing agent 7 is added to the crude (phthalic anhydride, the mixture can be heate to a temperature substantially less than its boiling point until condensation of the volatile impurities is com-- pleted. In some cases it will be advantageous to allow the heated .moltemmass to stand until at least a part of the impurities have settled out, before conveying the phthalic anhydride to the still or sublimer.

I claim: 1. The process of purifying crude phthalic 'anhydride prepared by the vapor phase catalytic oxidation of naphthalene, which comprises treating it with a condensing agent comprising an acidic material having an S0,

group, and vaporizing the phthalic anhydride from the relatively non-volatile impurities.

2. The process of purifying crude phthalic anhydride prepared by the vapor phase catalytic oxidation of naphthalene, which comprises treating it with a condensing agent com rising an acidic material having an [1 HS 4 grou and vaporizing the phthalic anhydride rom the relatively non-volatile impurities.

3. The process of purifying crude phthalic anhydride prepared by the vapor phase catalytic oxidation of naphthalene, which comprises treating it with a condensing agent comprising sodium hydrogen sulphate, and vaporizing the phthalic anhydride from the relatively non-volatile impurities.

ll 4. The process of purifying crude phthalic anhydride prepared by the vapor phase'catalytic oxidation of naphthalene, which comprises treatingit with a condensing agent comprising an acidic material in the proportion of .25 to 1% having an S0 group, and

vaporizing the phthalic anhydride from th relatively non-volatile impurities. p

5. The process of purifying crude phthalic anhydride prepared by the vapor phase cata- 85 lytic oxidation of naphthalene, which comprises treating it with a condensing agent comprising an acidic material from the group consisting of acid sodium sulphate, acid potassium sulphate and sulphonic acids,

80 and vaporizing the phthalic anhydride from the relatively non-volatile im urities.

In testimony whereof, I a my signature.

PAUL (l. BOWERS. 

